Impact of Land Surface Processes on a Record-Breaking Rainfall Event on May 06-07, 2017, in Guangzhou, China

A record-breaking extreme rainfall event with a maximum rainfall amount over 24 h of 524.1 mm occurred in Guangzhou, China, on May 06-07, 2017. To study the impact of land surface processes on this extreme rainfall, two 21-member convective-permitting ensemble forecasts over South China were performed based on two land surface models (LSMs), Noah and Community Land Model (CLM), and 21 forecasting members of the Global Ensemble Forecast System (GEFS). The results showed that, in general, members using the Noah LSM could better simulate urban heat islands (UHIs) and urban convection than members using the CLM LSM in this case. By investigating the ensemble member that most resembled the observations, it was found that the high temperature center in the urban area caused a thermal low in the early stage. As the southerly winds strengthened, the low-level convergence line continued moving northward and eventually triggered convection in the mountainous region. A sensitivity experiment showed that the impact of land surface heterogeneity on precipitation could be reflected on a finer scale, and heavy rainfall was very sensitive to the changes in small-scale land surface forces, including terrain and land use. Slight variations in small-scale land surface conditions caused great responses in the total precipitation, indicating that for the occurrence of such quasi-stationary extreme rainfall, a subtle balance between different atmospheric and land surface factors may be required.